# Applied Engineering: Exhaustive Search

In the last post of the series, we’ve introduced modules, functors and maps. We had ended with realization of the value assignment function that maps a given table assignment to a (ranking) value.

In this post, we will conclude the series and solve the optimal table assignment problem using OCaml. We will implement a function that searches through the space of potential table assignments to find (the) one with best possible value. We will use a very simple exhaustive search algorithm here.

First, we introduce a new module similar to the Map module we’ve already used previously. It models a set of objects and allows to add, remove, and find elements and to iterate over them efficiently. For our purposes, a set will be the right data structure for walking through the search space.

module IntSet = Set.Make(
struct
type t = int
let compare = compare
end
)

let int_list_to_set int_list =
List.fold_left (fun set_acc list_el -> IntSet.add list_el set_acc)
IntSet.empty int_list

As before, we have to specify a compare function, and we use the default one once again. We also add a function int_list_to_set that converts a list of integers to a set of integers by folding the list and adding one element at a time using IntSet.add while starting with the empty set IntSet.empty.

If we want to get the sets of people and seats, we could use the mapping function as follows:

let people_set = int_list_to_set (List.map (fun (x,_) -> x) people)
let seats_set = int_list_to_set (List.map (fun (x,_,_) -> x) table)

Before we can turn to the exhaustive search algorithm, we consider one last type that is particularly useful in functional programs: the option type. It is a box that can contain at most one instance of some other type – in other words, it’s either an empty box or a singleton. It is already defined by OCaml, but it can easily be redefined:

type 'a option = None | Some of 'a

You can use the type as follows:

let x = None
let y = Some 5
match option_variable with
None -> do_something
| Some i -> do_something_else

Back to our initial problem: finding an optimal table assignment. Just for fun, we will also try to find a worst table assignment, so the search algorithm should be given a comparison routine that allows the algorithm to select the more “appropriate” table assignment when comparing two.

We will therefore consider the following two comparison functions:

let better_selector assignment assignment' =
if assignment_value assignment > assignment_value assignment'
then assignment
else assignment'

let worse_selector assignment assignment' =
if assignment_value assignment > assignment_value assignment'
then assignment'
else assignment

Our exhaustive search algorithm will be called find_assignment, and it will be given one of the comparison routines as only parameter. We will then be able to create the following two convenience function searching for either a best or a worst assignment:

let best_assignment = find_assignment better_selector
let worst_assignment = find_assignment worse_selector

The basic idea of our exhaustive search algorithm is as follows. At any point in the algorithm we have four objects:

1. the currently “best” assignment (initially will be None),
2. a partial assignment (initially will be the empty map),
3. the set of empty seats  (initially will be the set of all seats), and
4. the set of persons without a seat (initially will be the set of all persons)

The algorithm will then proceed as follows. If there are no more people without a seat, the partial assignment (which is a full assignment at that point) will be compared with the currently best assignment, and the better one will be returned. If otherwise there are no more seats left, the algorithm will fail, because there are not enough seats for all persons. If there are still seats and persons left, the algorithm will pick the first person unseated and the first unoccupied seat, seat the person there, and recursively find the best assignment. Then, the algorithm will free the seat again, and seat the second person on the same seat and recursively find the best assignment again. This will be done for all persons unseated, so in the end, we’ll find a best assignment.

Here is the code:

let find_assignment selector =
let people_left = int_list_to_set (List.map (fun (x,_) -> x) people) in
let seats_left = int_list_to_set (List.map (fun (x,_,_) -> x) table) in
let rec find reference assignment people_left' seats_left' =
if IntSet.is_empty people_left' then
match reference with
None -> Some assignment
|   Some other_assignment -> Some (selector other_assignment assignment)
else if IntSet.is_empty seats_left' then
failwith "There are not enough seats!"
else
IntSet.fold (fun person reference' ->
let people_left'' = IntSet.remove person people_left' in
let seat = IntSet.choose seats_left' in
let seats_left'' = IntSet.remove seat seats_left' in
let assignment' = IntMap.add person seat assignment in
find reference' assignment' people_left'' seats_left''
) people_left' reference
in
match find None IntMap.empty people_left seats_left with
None -> failwith "There must be one assignment."
|   Some assignment -> assignment

So what do we get in the end? A best seating assignment gets a score of 7.5 and the assignment is:

Hank sits on seat #5
Karen sits on seat #4
Becka sits on seat #1
Mia sits on seat #2
Julian sits on seat #0
Trixi sits on seat #3

A worst seating assignment gets a score of -0.625 and the assignment is:

Hank sits on seat #0
Karen sits on seat #3
Becka sits on seat #1
Mia sits on seat #5
Julian sits on seat #4
Trixi sits on seat #2

# Applied Engineering: Modules, Functors and Maps

[latexpage]
In the last post of the series, we introduced the functional programming language OCaml and basic types like lists. We applied higher-order functions like map to them (recall that a higher-order function essentially is a function taking a function as an argument).

Recall that we’ve considered the list of people:

let people = [
(0, "Hank");
(1, "Karen");
(2, "Becka");
(3, "Mia");
(4, "Julian");
(5, "Trixi")
]

If we now want to look up the name of the person with id, let’s say, 4, we can only walk through the list until we find an entry that matches the respective number. This is a very inefficient algorithm for looking up data (its worst-case and average-case complexity are both linear). We can be much faster using a different data structure that stores all entries in order and applies binary search. The classic structure is a balanced tree and all database system rely on such organization of data. Luckily, OCaml already provides a module for handling such data. A module can be seen as a namespace for types and functions that belong together.

We will use the map module that allows to quickly look up a key and return a corresponding value – in our case, the keys would be the people’s ids and the values would be the people’s names. The map module is general enough to allow arbitrary keys and arbitrary values to be stored in a balanced tree.

If you think about that for a minute, you might see an issue here. In order to organize the data in the tree, you need to be able to compare keys with each other, so they can be arranged in the right order. But how would the map module compare stuff with each other if the map module doesn’t know how to compare certain types? Well, it doesn’t. We have to specify how our keys should be compared with each other.

How do we explain to the map module how our keys can be compared with each other? By parametrizing the map module by an auxiliary module that explains the key comparison. More precisely, we map the auxiliary module to a concrete map module by a functor (a mapping from a module to a module).

module IntMap = Map.Make(
struct
type t = int
let compare = compare
end
);;

On other words: we define the module IntMap to bethe  Map.Make-Functor applied to the auxiliary module with key type int (our people’s ids) and the comparator compare (OCaml provides a “magic” compare function for every type that we use here – for ints, it’s the natural ordering on integers).

Next, we want to convert our list of people to an IntMap, mapping ids to people’s names. In order to build and use the map, we need three IntMap-functions: one to create an initial empty IntMap-instance, one to add assignments to it and one to look up keys.

IntMap.empty: 'a IntMap.t
IntMap.add: int -> 'a -> 'a IntMap.t -> 'a IntMap.t
IntMap.find: int -> 'a IntMap.t -> 'a

Recall that ‘a is a type variable. In our case, the type variable corresponds to the value type – and since the map does not care about the specifics of this type, we don’t have to fix it via any auxiliary modules and functors.

The first function, IntMap.empty, simply returns an empty instance of an IntMap. The second function, IntMap.add, takes a key, a value and an existing IntMap as arguments and returns a new IntMap that added the new key-value assignment to the existing IntMap. The third function, IntMap.find, takes a key and an existing IntMap as arguments and returns the corresponding value (assuming that the key-value pair is present).

Our conversion function therefore looks as follows:

let int_list_to_map int_list =
let empty_map = IntMap.empty in
let rec helper rest_list acc_map =
match rest_list with
[] -> acc_map
|	(domain, range)::rest_list' ->
helper rest_list' (IntMap.add domain range acc_map)
in
helper int_list empty_map

The function starts with an empty map and then walks recursively through the last, adding the assignments one by one to the map. We can now use it to generate our people mapping and use it to look up people’s names:

let people_map = int_list_to_map people
let people_func p = IntMap.find p people_map

Recall that in our initial post, we had two more lists – the list of constraints (which person likes to sit next to which person) and the table configuration (which is seat is close to which set). Every entry in each of these lists is a triple – two ids and a value (how likable / how close).

let constraints = [
(0, 1, 1.0);
(0, 2, 1.0);
(0, 3, -0.5);
(0, 4, -1.0);
(1, 0, 0.75);
(1, 2, 1.0);
(1, 3, 0.5);
(1, 4, 0.5);
(1, 5, -0.75);
(2, 0, 0.5);
(2, 1, 0.5);
(2, 3, 0.75);
(2, 4, -0.75);
(3, 0, 1.0);
(3, 5, 0.5);
(4, 1, 0.5);
(5, 0, 1.0);
(5, 1, -0.5)
]

let table = [
(0, 1, 1.0);
(0, 4, 1.0);
(1, 0, 1.0);
(1, 4, 1.0);
(1, 2, 1.0);
(1, 5, 0.5);
(2, 3, 1.0);
(2, 1, 1.0);
(2, 5, 1.0);
(2, 4, 0.5);
(3, 2, 1.0);
(3, 5, 1.0);
(4, 0, 1.0);
(4, 1, 1.0);
(4, 5, 1.0);
(4, 2, 0.5);
(5, 3, 1.0);
(5, 4, 1.0);
(5, 3, 1.0);
(5, 1, 0.5)
]

We again want to convert both lists to maps, but in this case, the key consists of two ids, i.e. is an int product-type. We need to create a new module IntMap2 by mapping a new auxiliary module (that allows to compare key pairs with each other) to a map:

module Int2Map = Map.Make(
struct
type t = int * int
let compare = compare
end
)

let int2_list_to_map int2_list =
let empty_map = Int2Map.empty in
let rec helper rest_list acc_map =
match rest_list with
[] -> acc_map
|	(domain, domain2, range)::rest_list' ->
helper rest_list' (Int2Map.add (domain, domain2) range acc_map)
in
helper int2_list empty_map

let constraint_map = int2_list_to_map constraints

let table_map = int2_list_to_map table

We again want to define functions to look up the respective values by key pairs. There will be cases, however, in which certain key pairs don’t exist in the mappings. We didn’t specify, for instance, any constraints for the two seats 0 and 2 with respect to each other, because we implicitly assign them the proximity value 0. Similarly, this holds true for the constraints. We need to make sure, therefore, that our look up routines catch the exception in which no key pair could be found.

let constraint_func p q =
try
Int2Map.find (p, q) constraint_map
with
Not_found -> 0.0

let table_map = int2_list_to_map table

let table_func p q =
try
Int2Map.find (p, q) table_map
with
Not_found -> 0.0

We say here, try to execute the look up code provided by the Map module. If it raises the exception Not_found, return 0.

Next, we need to define a new type for handling dinner table assignments. It should be a map from a person’s id to a seat id – in other words an int IntMap.t. To play around with it, let us setup a test assignment that assigns each person the seat with the same id:

let test_assignment = int_list_to_map [(0,0); (1,1); (2,2); (3,3); (4,4); (5,5)]

So far, so good. To get a feeling of what’s going on, let us print the table assignment. We will make use of another higher-order function provided by the Map module that allows us to walk through the map:

IntMap.fold: (int -> 'a -> 'b -> 'b) -> 'a IntMap.t -> 'b -> 'b

This looks pretty complicated, so let’s check the details. The first argument is function that accepts a key and a value, and maps some data (of type ‘b) to updated data (of type ‘b again). The second argument is the map and third argument is some initial data (of type ‘b). Overall, the routine routines data of type ‘b. Internally, the function starts with initial data and the first key-value-pair and calls the user function to obtain an updated data object. It continues in that fashion with the second key-value-pair until all key-value-pairs have been handled. The final updated data object is then returned.

We will make us of it build a textual representation of dinner table assignments:

let format_assignment assignment =
IntMap.fold (fun person seat acc_format ->
acc_format ^
people_func person ^
" sits on seat #" ^
string_of_int seat ^ "\n"
) assignment ""

How does it work? It starts with the empty string “” and adds a line for every assignment pair in the user defined function. The current string is given by the accumulator variable acc_format. The output accumulator is built by concatenating (denoted by ^ in Ocaml) the accumulator variable with the person’s name and the seat number.

Applying that to our test assignment, we get the following output:

# print_string (format_assignment test_assignment);;
Hank sits on seat #0
Karen sits on seat #1
Becka sits on seat #2
Mia sits on seat #3
Julian sits on seat #4
Trixi sits on seat #5

We conclude the post by realizing the assignment value function of our initial post that allows us to rate a given dinner table assignment:
$\sum_{p,q \in People} constraint(p,q) \cdot proximity(assign(p), assign(q))$
We make extensive use of the fold operation again (which corresponds to the sum operation here):

let assignment_value assignment =
IntMap.fold (fun p _ acc_value ->
IntMap.fold (fun q _ acc_value' ->
acc_value' +. constraint_func p q *.
table_func (IntMap.find p assignment)
(IntMap.find q assignment)
) people_map acc_value
) people_map 0.0

For the record, the assignment value of our test assignment is 3.5. We should find an assignment that yields a higher value, right? We will see how to accomplish that in our final post in the series.

# Applied Engineering: Types, Lists and Functions

In the last post, we introduced the problem of assigning seats at your dinner table to your guests in an optimal way – and by optimal, we mean that most constraints can be satisfied most accurately.

In order to solve the problem, we use the functional programming language OCaml. Functional programs are very close to mathematical formulations – it is about the definition of data and functions operating on data and not so much about how to compute stuff with it. So let us define the list of people first. Every person is described by a number and his or her name – mathematically, we describe that by a pair, which is exactly what happens in OCaml:
 (0, "Hank") 

Ocaml is a typed language, so it will tell you what type the definition you just made has. The type corresponds to the set-theoretic universe in which the pair lives. Integers like 0 have the type int while words like “Hank” have the type string. Therefore Ocaml will infer the following type (you can try that out by starting “ocaml” in your terminal and entering the pair terminated by “;;”):
 - : int * string = (0, "Hank") 
That makes sense – a pair (“*”) of an integer and a string.

The description of the whole table is a different story – in general, we have an arbitrary number of people. Mathematically, we could describe that by a set – a similar type exists in functional languages as well. But we first introduce a simpler type: the list. A list contains an arbitrary number of objects of the same type in a fixed order. In Ocaml, we introduce the list of persons as follows:
 let people = [ (0, "Hank"); (1, "Karen"); (2, "Becka"); (3, "Mia"); (4, "Julian"); (5, "Trixi") ] 

Again, Ocaml infers the type which is a list of pairs:
 val people : (int * string) list 

Next, we want to crunch some numbers. For starters, let’s try to find out how many people there are in the list. As a mathematician, one would define a recursive function that sums up the number of objects in the list – and that’s exactly what we will do in a functional language:
 let rec cardinal list = match list with [] -> 0 | obj::list_rest -> 1 + cardinal list_rest 
The first line introduces a definition again – we define a recursive function named “cardinal” that has one argument named “list”. The function is defined by a pattern matching on the given list. If the list is empty “[]”, the function returns 0. Otherwise, the list can be partitioned into a head named “obj” and a tail named “list_rest”, and the number of items in the list can be computed by counting the number of items in the the tail of the list plus 1.

Ocaml gives our function “cardinal” a type again:
 val cardinal : 'a list -> int 
Therefore “cardinal” is a function that maps a list based on a type variable ‘a to an integer. The type variable means that “cardinal” does not depend on how the objects of the list look like. Indeed, we can apply “cardinal” to our list of people:
 cardinal people: int = 6 

Let us consider another example of a recursive definition. We could be interested in obtaining the list of names without the integers. For this, we actually have to solve two problems: obtaining an element of a pair and mapping every object of a list to a new object. In order to select the second element of a pair, we use pattern matching again:
 let second (x,y) = y 
Ocaml infers the following type:
 val second : 'a * 'b -> 'b 
Therefore, “second” is a function that takes a pair with type variables ‘a and ‘b and maps it to the type variable ‘b. In other words, the function does not care about the type of the first entry of the pair and preserves the type of the second entry of the pair.

We continue with mapping a list of objects to a new list of new objects. For this, we assume that “f” is a function that maps an object to a new object. Then, we can define the mapping operation as follows:
 let rec map f list = match list with [] -> [] | obj::list_rest -> (f obj)::(map f list_rest) 
As before, we define “map” to be a recursive function that takes “f” and “list” as arguments. If “list” is an emtpy list, it returns an empty list as well. Otherwise, the list can be partitioned into a head “obj” and a tail “list_rest”. We apply “f” to “obj” in order to get a new object and use it as head of our new list that we create by calling “map” recursively on the tail of the list.

Ocaml infers the following type:
 val map : ('a -> 'b) -> 'a list -> 'b list 
The type might look scary at first sight – it says the following: “map” is a function that maps the argument “f” to a function that maps the argument “list” to a result. The first argument “f” has the type (‘a -> ‘b) which requires “f” itself to be a function that maps something of type ‘a to something of type ‘b. Then, “map” maps a list with objects of type ‘a to a list with objects of type ‘b.

We can now apply “map” to our function “second” that selects the second entry of a pair:
 map second: ('a * 'b) list -> 'b list 
In other words, “map second” is now a function that takes a list of object pairs and maps it to a list of objects that share the type of the second pair items of the original list. If we now apply this to our list of people, we get the following result:
 map second people: string list = ["Hank"; "Karen"; "Becka"; "Mia"; "Julian"; "Trixi"] 

In our next post in the series, we will consider advanced types like Sets and Maps, and introduce a type for describing table assignments, bringing us closer to the solution of our problem.

# Applied Engineering: Functional Languages and the Dinner Table Problem

[latexpage]
This will be the first post in a series called “applied engineering”. It will feature small tutorials on engineering problems as well as gentle introductions into the art of software engineering. Even non-tech people should be able to follow the series.

My first post in the series considers functional languages, a class of programming languages. Functional languages are mostly used in scientific environments but are now gaining traction in industry applications as well. I will add a couple of posts on functional languages to get the interested reader started.

As with all language paradigma, there is a bunch of concrete realisations that we can use. For our purposes here, we use Ocaml – so if you want to try our little snippets, go ahead and install Ocaml on your own machine.

Now what exactly are language paradigma and what are the different programming languages? Language paradigma are based on the same idea as language families in natural languages – you group languages together that share the same roots. The main language paradigma that we have in computer science are imperative languages, functional languages, markup languages, logic languages and object orientation, although object orientation could be seen as a “language topping”. It is mostly but not exclusively used in combination with imperative languages though.

So we are using the functional language called OCaml. We could also use Haskell – we would stay in the same language family (like Germanic languages). In other words, once you’ve understood the concepts of functional languages using Ocaml, you can easily adapt to Haskell in case you want to.

Let’s start with a real-world problem to see how we could solve it using Ocaml. Assume that you host a dinner party and you’re having a hard time assigning people to your table as some people want to sit right next to each other while others cannot without ruining your evening. After a couple of minutes writing down some possible seatings on a piece of paper, you give up – there are just too many combinations.

Hence let the computer solve the problem for us by using our Ocaml program – because that’s what programs are all about: you give them some well-structured data – in our case whether people want or not want to sit next to each other – and let them compute a solution to the data – in our case a favourable dinner table seating assignment.

Before starting to write a single line of code, let us structure the data. For the purpose of this example, we need to add a bunch of people, a bunch of constraints (who wants to sit or not sit next to whom) and some table configuration (i.e. what seats are next to each other).

First, let us start with the people. We will assign consecutive natural numbers starting from 0 to all people:
 0 Hank 1 Karen 2 Becka 3 Mia 4 Julian 5 Trixi 
Second, let us add some constraints. A constraint will be of the following form:
 Person A likes Person B by +/-p% 
This means that person A would like to sit next to person B by a specific rating – ranging from 0% – 100%. If person A would rather not sit next to person B, we let p% range from -100% to 0%. Note that person A wanting to sit next to person B does not necessarily imply the converse. Let us assume the following constraints:
 Hank likes Karen by 100% Hank likes Becka by 100% Hank likes Mia by -50% Hank likes Julian by -100% Karen likes Hank by 75% Karen likes Becka by 100% Karen likes Mia by 50% Karen likes Julian by 50% Karen likes Trixi by -75% Becka likes Hank by 50% Becka likes Karen by 50% Becka likes Mia by 75% Becka likes Julian by -75% Mia likes Hank by 100% Mia likes Trixi by 50% Julian likes Karen by 50% Trixi likes Hank by 100% Trixi likes Karen by -50% 
This for instance tells us that while Hank isn’t too eager to sit next to Mia, she, on the other hand, would like so very much.

Third, let us add some table configuration. We do this be specifying the proximity between seats while no proximity specification means that these two seats are so distant from each other that the two people sitting there could not be considered next to each other. For our purposes, we assume a small dinner table with one seat on each end and two seats on each side. We will give every seat a number again, assigning 0 and 3 to the left resp. the right end, and 1-2 resp. 4-5 to the bottom resp. top side. We will give the proximity specification by such lines:
 i j p% 
This means that seat $i$ is in $p%$-proximity of $j$ where $100%$ means directly next to each other and $50%$ for instance means that these two chairs are near each other but not exactly next to each other. For our table, the specification could look as follows:
 0 1 100% 0 4 100% 1 0 100% 1 4 100% 1 2 100% 1 5 50% 2 3 100% 2 1 100% 2 5 100% 2 4 50% 3 2 100% 3 5 100% 4 0 100% 4 1 100% 4 5 100% 4 2 50% 5 3 100% 5 4 100% 5 3 100% 5 1 50% 
This table tells us, for instance, that the bottom left seat 1 is right next to left head seat 0, bottom right seat 2 and top left seat 2, while it is only “near” top right seat 5 and not at all next to the right head seat 3.

A dinner table seating assignment is now an identification like “Hank sits on seat number 3 and Karen sits on seat number 0”. And our program will search for such an assignment that optimizes all given constraints. We will use a mathematical formulation to explain what we mean by optimal – intuitively, we want to maximize the accumulated satisfaction of constraints.

More formally, let $assign: People \rightarrow Seat$ be a table seating assignment, $proximity: Seat \times Seat \rightarrow Precentage$ be the proximity configuration between two seats and let $constraint: People \times People \rightarrow Percentage$ be the constraint of two people. We then want to maximize the value of
$\sum_{p,q \in People} constraint(p,q) \cdot proximity(assign(p), assign(q))$
with respect to the table seating assignment. In other words we build the sum over all pairs of people, where the value of a pair is given by the product of its proximity and its constraint, i.e. the further away two people are the less relevant the constraint is.

Are we still on the same page? You had this or similar problems before? Great! You have just seen how you model a real-world problem in terms of mathematics and processable structures. In the next post, we will write our first Ocaml program and integrate this data.